I think the issue is spines are actually more recessed, with the thickness of the back more around the actual shoulder joint. The center of the back should be more concave, where in your model you have it bulging outward too much.

Ha, you can't really get around anatomy with characters (unless you only draw mechanical things, like some crazy people...). It's one of those things where it's way easier to see that something is wrong than know what is right.